1. Field of the Art
This invention relates to a sustained-release granule which releases sugars of a low molecular weight depending on the temperature conditions of circumferential water, particularly the granule suitable for supplying nutrients to a plant meristematic tissue in an artificial seed.
2. Background Art
Recently, there have been successfully obtained plant bodies having a uniform and excellent character by the use of plant meristematic tissues such as a somatic embryo, an adventitious bud, a shoot primordium and a callus which are obtained by culturing a part of a tissue of a plant body. It is necessary to use such plant meristematic tissues in a seed analogue so that they will handle easily and can be stable physicochemically and biologically, and a sufficient supply of nutrients to them will be ensured until they grow by themselves by ingesting nutrients from their surrounding circumstances, in order to efficiently germinate, root and grow the plant meristematic tissues and to transport and plant in a green house or in a farm the plant meristematic tissues which have been cultured in a large scale in a factory. Such seed analogues containing the plant meristematic tissues are called artificial seeds, and some of them are specifically disclosed in, for example, Japanese Patent Laid-Open Publication Nos. 102308/84, 118103/85 and 2314811/85. The artificial seed is the product in which a plant meristematic tissue is encapsulated in a gel.
If a carbohydrate as an energy source is incorporated in a gel in the form of a high-molecular weight compound such as starch like the case of natural seeds in order to incorporate nutrients which correspond to endosperm into a gel, a plant meristematic tissue which lacks in an enzyme which decomposes such a high-molecular weight compound cannot grow by utilizing it. On the other hand, if a carbohydrate is incorporated in the gel in the form of a low-molecular weight compound such as sucrose or glucose for its easy and direct utilization and added in an amount sufficient to provide the whole nutrient needed for the growth of a plant meristematic tissue until it can ingest nutrients by itself from surrounding circumstances, the osmotic pressure becomes higher and water moves from the plant meristematic tissues into the gel so that the plant tissue will lower its growth and sometimes wither. For this reason, it is required to have a composition which will not release a nutrient of a low-molecular weight compound into the gel during storage but will gradually release the nutrient in 2 to 4 weeks after sowing, ideally a composition provided with a characteristic that the nutrient is supplied into the gel in such a manner that a suitable amount will be gradually supplied depending on the growth of a plant meristematic tissue. As to control of release of nutrients in an artificial seed, a method of using an adjuvant in the form of microcapsule is disclosed in Japanese Patent Laid-Open Publication No. 214811/85 which, however, only suggests a possibility and does not teach anything about the necessary properties or qualities regarding the microcapsule. Two examples of developments on microcapsules for an artificial seed are disclosed in "Hort. Science". 22, No. 5, 803-809 (1987), but a sustained-release of nutrients is not yet achieved in these examples.
On the other hand, the so-called "slow-acting fertilizer" which gradually releases fertilizer components from it scattered on a farm has been put to practical use, and it is described for example in Japanese Patent Publication No. 20371/69 that such a fertilizer is prepared by coating a mixture of an ethylene-vinyl acetate copolymer and a paraffin o fertilizer particles. However, the slow-acting fertilizers are designed to be stored in a dry state and gradually release fertilizer components after they are scattered and wetted. Thus, these fertilizers are not intended for their storage in a wet state and their release of fertilizer components which is controlled by temperature. Accordingly, these fertilizers do not give any suggestion as to whether it is possible to control release of contents by temperature.